The invention relates to a process for producing molded articles from polylactams by activated anionic polymerization of medium lactams. By the term "medium lactams" used herein, lactams having from 12 to 15 C-atoms in the lactam ring are referred to, i.e. the lactam of n-tridecane myristin and n-pentadecanoic acid as well as that of the lauric acid, which is particularly preferred.
It is known that in the polymerization of lactams, the monomer lactam is melted and the melt is mixed with at least one catalyst and at least one activator, whereupon the polymerization reaction takes place, for which it is already known, too, from Swiss Pat. No. 479 654 and German laid-open application No. 2 108 759, to carry out the polymerization in a manner such, that a partial quantity of the lactam melt is actively mixed with the catalyst and the other partial quantity with the activator.
From U.S. Pat. No. 3,793,,255 in addition a process for producing molded articles by the activated anionic polymerization of lauric-lactam is known, wherein two lactam melts are separately actively mixed with a catalyst and an activator, with nitrogen acting on each melt and are fed to the mixing device via separate tube systems being under the same temperature action as the melts, and, thereupon, flow into a mold, with the thus treated partial melts being urged to pass through one siphon-like spiral tube system each on their way to the mixing device. By this process qualities of the polymerization product (polymer) are obtained which are completely different from those of the common polycaprolactam.
The drawback (disadvantage) of this process, however, resides in the fact, that in the beginning (of the process) polymerized molded products of excellent qualities may be obtained, that, however, these qualities are very much deteriorated already upon a short interval of time, and that finally, in numerous cases, products with insufficient qualities are obtained.
A modification of this process for producing molded articles from polylactams is suggested in German laid-open application No. 25 07 549. By this process, for instance, a polylaurin-lactam is obtained from lauric-lactam, having the following properties completely different from those of the known polycaprolactams:
a yield sress .sigma..sub.S of about 470 to about 520 kp/cm.sup.2, measured according to DIN standards 53 455;
a tension at yield stress .epsilon..sub.5 of about 17 to about 25% measured according to DIN 53 455;
a tensile strength .sigma..sub.R of about 500 to about 630 kp/cm.sup.2, measured according to ISO R 527;
a breaking tension .epsilon..sub.R of about 200 to 350% measured according to ISO R 527;
a modulus of elasticity E of about 19 000 to about 22 000 kp/cm.sup.2 measured according to DIN 53 457 para 2.3,
a critical bending stress .sigma..sub.B of about 750 to about 1000 kp/cm.sup.2 measured according to ISO R 178;
a notch impact strength a.sub.k of about 55 to 65 kpcm/cm.sup.2 measured according to DIN 53 453;
a ball pressure hardness of 10" of about 1000 to about 1050 kp/cm.sup.2, measured according to DIN 53 456 step C;
an abrasive resistance of about 158 to about 129 mm.sup.3 /rpm measured by means of Taber-Abraser;
a time yield stress .sigma..sub.1 /1000 (23.degree. C./95%) of about 50 to about 60 kp/cm.sup.2 measured according to DIN 53 444;
a modulus of creeping E.sub.c /1000 (.sigma.20,0) of about 13 000 to about 14 000 kp/cm.sup.2 measured according to DIN 53 444.
However, this process, too, is not free of a deterioration encountered in the course of a longer pot time, with by "pot time" the interval of time being referred to, during which the partial melt containing the catalyst and the partial melt containing the activator are kept separated from each other prior to the mixing and molding (casting).
The reason for this unsatisfactory behaviour, on the one hand, and the rather quick failure in obtaining optimum qualities of the products in the course of the operation time, is not readily evident. Yet, in the scope of the extensive tests carried out in regard of the present invention, it was found, that, on the one hand, the lactam melt, containing the catalyst, is more viscous than the lactam melt containing the activator, so that thereby already a non-uniform outflow of the separated melts from their vessels and a non-uniform supply to the mixing area are effected. Attempts to obtain a more uniform viscosity of the two partial melts by raising or lowering, respectively, the temperatures of the individual melts failed, too, since thereby the most favorable temperature required for a uniform injection molding was modified and, accordingly, inferior products were obtained. In these tests, it was additionally found, that with respect to the two partial melts and the temperature at the injection nozzle or mouth piece very special temperature conditions have to be met, in order to obtain acceptable results at all. Even when duly taking into account the two aforementioned aspects, it was found, however, that this was not yet sufficient, in order to obtain a satisfactory product, but that for this purpose, it is additionally required that part of the gases which develop or are encountered in the mixing zone and in the partial polymerization taking place therein, have to be removed from the mixing zone and returned to the vessels.
One problem of the invention, therefore, resides in providing a process for producing molded articles from polylactams, for which properties different from the known polycaprolactam may be obtained throughout the operation period and wherein the above-identified disadvantages are not encountered.
It is a further problem of the present invention to provide an apparatus suitable for carrying out such process.